The invention relates to a method for operating a field-oriented rotating-field machine supplied by a controlled converter.
In the following, the voltages or currents at the inputs of the rotating-field machine as well as the flux are designated by vectors with two defining quantities in a mathematical coordinate system. A stator coordinate system fixed in space which characterizes the stator winding has the Cartesian axes .alpha.1 and .alpha.2. A corresponding polar coordinate system describes a vector by the amount and direction ("angle"). For instance, the flux vector with the amount .PSI. and the angle .phi.s relative to the .alpha.1 axis in the Cartesian stator coordinate system by a pair of numbers .PSI.=(.PSI.cos .phi.s, .PSI.sin .phi.s). By means of .phi.s, the axis .phi.1 parallel to the field and the axis .phi.2 perpendicular to the field of a field coordinate system is then also fixed.
The correlation of the two defining quantities (components) of the current and voltage vector with the quantities appearing at three stator windings is accomplished by means of 2/3 and 3/2 converters, respectively. Angles are preferably likewise processed as unit vectors; for instance, the vectors .phi.s=(cos .phi.s, sin .phi.s) is obtained for the field axis (.phi.1-axis) in the stator coordinate system. It can be calculated from the flux vector by a vector analyzer "VA". A vector can be transformed by means of a vector rotator "VD" from the stator coordinate system into the field coordinate system or vice versa, the required transformation angle, however, being equal to the field angle.
It is a feature of field-oriented operation of the preferably used asynchronous machine that the control input or reference values for the stator current vector according to FIG. 1, are set by a reference value transmitter RV as the vector i.phi.* in the field coordinate system and the converter with stator-oriented control variables which correspond to a control vector is* and are brought to a value given by the control inputs by a current controller IR (individual controllers for the components of the vector or for the phase currents transformed into the three-system). A condition for the control is, however, that the vector difference to be eliminated is formed by vectors which are shown in the same respective coordinate system.
In the arrangement according to FIG. 1 at least the angle .phi.s is formed in the stator coordinate system from the current vector is and the voltage vector us by means of a flux computer FC. This can preferably be accomplished by the provision that the voltage vector us, after deducting the ohmic drop rs. is is vectorially integrated and the flux vector .PSI.s is formed by deducting the integral inductive voltage drop xs.multidot.is from which a vector analyzer VA calculates .phi.s.
The field oriented control device VC operates here in the field coordinate system, where in the other switch position not shown in FIG. 1, of the switching device SW1 and SW2, the vector rotator VD1 transforms the stator current vector is into the field coordinate system (vector i.phi.) and the quantities formed by the current controller IR from the control deviations of the components are transformed by means of the vector rotator VD2 into components of the stator-oriented control vector is* for addressing the control unit ST.
The flux computer FC which ultimately forms the flux .PSI.s as the integral of a corresponding vector es of the EMF operates satisfactorily only if at sufficiently high frequencies, the induced voltages are superimposed by relatively small disturbances. In addition, the integrators INT of the flux computer must be set to the value of the true flux components already when the machine is started up, if the advantages of field-oriented control are to be achieved.
It is known from European Patent No. E-B1-0015 501 to use for this purpose the setting means E1 and E2 shown in FIG. 1 as well as the switching device SW1 and SW2. Apparatus developed according to this principle for operating a rotating-field machine thus comprises a flux computer which calculates from the electrical measurement values of the machine, the field angle of the machine and thereby the direction angle of one axis of a Cartesian field coordinate system, and a control device with a setting input for the field angle, with means for presetting control variables for the components of the stator current vector in the field coordinate system, and with a transformation device which furnishes from the control quantities and the field angle, stator-oriented control quantities for the converter. For the start, a start input device is provided which presets by means of E1 an initial direction .phi.MO* for the flux vector of the rotating field machine which replaces the field angle .phi.s, and by means of E2, starting values iO*.noteq.0 and iM*=0 for the control quantities i.phi.1* and i.phi.2* of the vector i.phi.*. The switching device connected to the flux computer, the control device and the start input device releases, in preparation of starting the machine, the starting direction and the starting values and blocks them in order to release the starting of the machine.
The procedure here is as follow:
(a) in a first step, with the machine standing still, the starting angle .phi.MO* is set-in which determines the direction of a model flux vector and an axis M.phi.1 of a model coordinate system parallel thereto. Further preset are the input variables iO*.noteq.0 and iM*=0 which thus preset an input vector which is thus parallel to M.phi.1, is designated with iO* and which is transformed by means of transformation angle derived from the starting angle, namely, the starting angle .phi.MO* itself, into the vector is*, i.e. into the stator-oriented control quantities for the converter. During this time, the line switch SWN is closed so that the electronic control circuitry is connected to voltage while the converter itself is still blocked by the open switch SWS. The first step ends with the closing of SWS, whereby the converter is released for exciting the machine.
(b) In a second step, the converter is now controlled by the stator-oriented control variables which in this step determine a constant current vector is*, so that a constant flux which is defined by the direction of is=iO* and points in the direction of M.phi.1 or .phi.MO* is being built up in the machine, for the true field angle .phi.0 of which the value .phi.s is calculated from corresponding electrical parameters fixing the field angle of the machine. This calculated field angle .phi.s then fixes a field coordinate system coinciding with M.phi.1 and M.phi.2. (FIG. 2)
(c) In a last step a switching operation is then performed by the control vector iO* to control variables i.phi.1*, i.phi.2* of the vector i.phi.* depending on the operation, and the control variables are transformed into the stator-oriented control variables is* for the converter by means of the calculated field angle .phi.s. The machine then starts up in normal operation.
This method eliminates the mentioned disadvantages only if the flux computer images the true field angle .phi.0 also correctly into the calculated field angle .phi.s. This, however, is not assured.